This invention relates to feed apparatus for machine tools, particularly for automatic lathes, of the type having bearing units, each one of which includes a circular passage receiving an injection of oil, intended for a machine working a material supplied to it in the form of a bar capable of being driven rotatingly at high speed by the machine and which is pushed by a pusher having a carrier.
U.K. Published Patent Application No. 2,080,160 proposes feed apparatus of the aforementioned type made by disposing one behind the other bearing plates suitable for guiding a bar of material to be machined. In this apparatus, the bearing plates are axially movable during operation, and the unit which they form contracts in the manner of an accordion as the bar advances. When all the bearing plates are pushed together toward the front, in the direction of the machine tool, it is cumbersome to introduce a fresh piece of bar stock; moreover, the part forming the carrier fixed to the rearward end of the pusher (bearing plate 28d in this U.K. publication) need never pass a bearing plate because these plates recede before the carrier as it advances. It has been noticed, however, that in such a design, the bearing plates cannot be held very firmly on a frame since they must be able to slide axially. Considering the development of machine tools and the performance which this development demands of feed apparatus, as will be explained below, it would be desirable to have feed apparatus of the type initially mentioned in which the bearings are fixed firmly and non-displaceably on the support frame of the feed apparatus during operation. Fixing of the bearing plates would involve numerous problems, however, relating to the introduction of a fresh or subsequent piece of bar stock and to the advance of the carrier. It would, in fact, be necessary to have available at the rear of the feed apparatus a free space as large as the apparatus itself, first for running the bar in, then for providing the space needed for a pusher which ought to be as long as the bar of material.
The prior art also includes hydrostatic bearings, the theory of which has been thoroughly examined, particularly in an article by Professor Ichiro Inasaki of Hiyoshi, Japan, which appeared in the journal Werkstatt und Betrieb, No. 111 (1978), page 9, entitled "Optimierung hydrostatischer Axelager bei Werkzeugmaschinen." However, such previous studies and reports hardly considered the very special case of feed apparatus in which a bar of material, continuously replaced during operation, must rotate at high speed at the same time as it must advance through the feed apparatus.
U.S. Pat. No. 3,145,513 relates to a bearing supporting a workpiece rotating at high speed. There, however, it is primarily a question of rotatingly supporting camshafts during grinding or truing, which does not involve any axial displacement of the shaft in the bearing. The bearing disclosed in this U.S. patent is of the hydrostatic type, and the specification shows that it may be opened laterally (by sections 40, 41). However, this disclosure is remote from the problem of feed apparatus; for although lateral opening of the bearing is provided for positioning a camshaft, there is no provision (nor any reason) for satisfying the special requirements of feed apparatus, particularly the requirement for sequential operation of the bearing units, considering that some of them (at the front) must also reliably perform their function of hydrostatic rotary guidance, whereas one or more other bearing units (at the rear) must allow the carrier to pass. Very particular means for controlling the various bearing units must therefore be provided in the case of feed apparatus in order to satisfy conditions not encountered in U.S. Pat. No. 3,145,513.
In this context, it is important to note that in the field of machine tools, and especially that of automatic lathes, the speed of rotation of the bar stock has hitherto been determined by the maximum linear speed permitted by the cutting tools machining the bar, as a result of which large-diameter bars have rotated at a relatively moderate speed, whereas very high speeds of rotations have been applied only to small-diameter bars. The feed apparatus for automatic lathes proposed until now has allowed such speeds of rotation of the bar stock.
Recently, however, the situation has changed because of the advent of numerically controlled (NC) automatic machines, particularly automatic lathes, especially those having "computerized numerical control" (CNC). With such machines, the cutting speed permitted by the commercially available tools (currently some hundreds of meters per minute) tends to be utilized to the maximum in all machining situations, meaning that when machining an end nib 10 mm. in diameter on such an automatic lathe from a piece of bar stock 50 mm. in diameter, for example, the tendency is to rotate the bar so as to maintain the high cutting speed even then, which means that the speed of rotation is no longer limited by the diameter of the bar stock, in conjunction with the maximum linear cutting speed permitted by the machining tools. Briefly stated, the nearer the turning tool comes to the center, the faster the bar should rotate, even if it is of large diameter. In the example above, taking the diameter of the bar into account, this means a speed of rotation five times that ordinarily used in the past.
The development of feed apparatus for machine tools, particularly automatic latches, has difficulty keeping up with this technical evolution. A good solution for doing so is proposed by the present invention, having recourse to hydrostatic bearing technology. With hydrostatic bearings, however, the same difficulties are encountered with respect to space requirements and the passing of the carrier as have been mentioned earlier in connection with U.K. Pat. No. 2,080,160. It should be noted that the problem of space is solved on the feed apparatus known as "Hydrobar" and "Super-Hydrobar" marketed by the present assignee, where lateral displacement makes it possible to insert a fresh piece of bar stock from the space alongside the lathe. As the pusher is pushed in that apparatus by hydraulic pressure rather than by means of a carrier, the passage of a carrier is no problem. The aforementioned U.S. Pat. No. 3,145,513 provides, within the framework of the use of hydrostatic bearings, the beginning of a solution relating to the loading space, i.e., the free space needed for loading a fresh bar. However, this prior disclosure does not provide any solution to the problem posed by the passage of the carrier through some bearing units while other such bearing units must maintain their full hydrostatic bearing effect. Moreover, according to the teachings of this U.S. patent, the clearance between the rotary inner part and the non-rotary outer part of the hydraulic bearing is at most a few hundredths of a millimeter, preferably a few microns. Now, the tolerance in the diameter of bar stock does not allow the provision of so small a clearance in the case of feed apparatus, where each bar slides along its length through the bearing, and where the bars are frequently replaced. Use in feed apparatus requires that a larger hydrostatic interstice be provided, in a manner not obvious from the prior art.
Thus, in endeavoring to produce hydrostatic bearing-unit feed apparatus of this type, numerous problems are encountered which have not been solved by the prior art. The present invention aims in general at providing feed apparatus with a very high speed of rotation of the bar stock, by means of which a solution may be found to the problems discussed above.
A further problem, likewise solved according to a particular aspect of this invention, consists in the use of bar stock having a polygonal, especially hexagonal, cross-section. It is obvious to provide for that purpose a rotary part having a matching polygonal inner shape, but then lateral opening of the bearing unit for insertion of a bar and/or for the passage of a carrier poses a problem which is difficult to solve. As it happens, the prior art, and more specifically U.S. Pat. No. 3,135,533, which is the only one to provide for a hydrostatic bearing opening laterally, does not propose or even suggest any solution. The provision of feed apparatus of the type in question for bar stock of polygonal cross-section is likewise one of the important capabilities of the present invention.
Hence it is an object of this invention to provide improved feed apparatus for a machine tool, particularly for an automatic lathe, by means of which the drawbacks, inadequacies, and limitations known to the prior art are avoided.
A further object of this invention is to provide such feed apparatus capable of working under the particular conditions which tend to be imposed by the current development of machine tools.
Still another object of this invention is to provide feed apparatus capable of guiding a piece of bar stock rotating at very high speed even though it is of a considerable diameter; this is understood to mean a speed of rotation which may reach and even exceed 6,000 rpm or as much as 10,000 rpm with bars at least 60-80 mm. in diameter.
To this end, in the feed apparatus according to the present invention includes a series of spaced, fixed bearing units, the respective circular passages of which are axially aligned for the passage of the bar stock. Each bearing unit receives the injection of oil in order each to constitute the non-rotary part of a hydrostatic bearing, the rotary part of the latter being the bar stock itself or an intermediate rotary member through which the bar passes. The bearing units each comprising several unit portions, in each of which a part of the passage is formed, each unit portion thus including a concave wall forming, along an arc, part of the circumference of the passage. No individual arc exceeds a length on the order of half the circumference. The unit portions are capable of moving away from one another to provide a lateral opening, and control means are provided, in a cooperative relationship with the unit portions of the series of bearing units, to bring about successively for each bearing unit, at the moment when the carrier approaches it, such a lateral opening of a size at least sufficient to allow the carrier to pass. The control means likewise is capable of causing the unit portions in all the bearing units to move away from one another so as to open a large lateral access route by means of which a fresh or subsequent piece of bar stock can be laterally introduced, whereupon the unit portions are put back into end-to-end mutual contact so that their respective concave walls once more form the fixed part of a hydrostatic bearing for the fresh bar.
The advantage of the various preferred embodiments of the invention to be described and claimed hereinafter resides particularly in a design which is at once reliable, simple, and effective, in very efficient use of the particularities--for many still unexplored--of hydrostatic bearings, in the achievement under advantageous conditions of feed apparatus for automatic lathes attaining performances hitherto deemed virtually impossible, etc.
The phrase "arc not exceeding a length on the order of half the circumference" as used hereinafter shall be understood to mean that the arc may be less than 180.degree., equal to 180.degree., or even exceed 180.degree., but only to an extent such that the two C-shaped extremities thus formed allow a space to remain between them which is less than the maximum inside diameter only by a few percent. In other words, it is an arc which still permits the lateral introduction of an inside part, taking into account that this inside part has a diameter slightly less than the inside diameter of the part which includes the arc.
The phrase "on the order of 1 mm." as used hereinafter with reference to a difference in diameter between a non-rotary outside part and an inside rotary bar shall be understood to mean that this difference in diameter may range from a fraction of a millimeter (at least several tenths of a millimeter) to several millimeters (at most half a centimeter). In most cases, this value "on the order of 1 mm." will be between 0.5 and 2 mm.